Use of a feline cell line in the syncytia infectivity assay for the detection of bovine leukemia virus infection in cattle.

This report describes a modified syncytia infectivity assay (SIA) for the direct detection of bovine leukemia virus (BLV) in blood lymphocytes of cattle, using transformed feline (CC81) cells as the indicator system. The data show that the syncytia present in cultures of CC81 cells inoculated with BLV-infected cells are specific and arise through a mechanism similar to that responsible for the phenomenon of "late" polykaryocytosis described in other virus systems. The susceptibility of the CC81 cells to the syncytia-inducing effect of BLV-infected cells is comparable with that of early passages of bovine embryonic spleen cells, which were previously used as the indicator system in the SIA. Unlike the bovine embryonic spleen cells, CC81 cells retain their susceptibility to syncytia induction for long periods of cultivation. Furthermore, the syncytia induced in the CC81 cultures are larger and easier to identify. Thus, the CC81 cells can be used advantageously as the indicator system when the SIA is applied to the detection of BLV-infected lymphocytes. The results of the SIA for the detection of infective BLV agreed closely with those of the radioimmunoassay for the detection of BLV antibodies in randomly examined cattle. On the other hand, many cattle in early stages of infection were positive in the radioimmunoassay several months before they reacted in the SIA. The detection of BLV in blood lymphocytes provides a useful method for the diagnosis of BLV infection in cattle when serologic tests cannot be used, eg, calves that may have passively acquired maternal antibodies and cattle given BLV vaccines.